Unable to send message;Reason:550 5.4.6 Mail rate exceeded limit

Unable to send message;Reason:550 5.4.6 Mail rate exceeded limit

Hi

Occasionally we send emails to all our customers, (around 90) and when I reach about 70 I get the message ' Unable to send message;Reason:550 5.4.6 Mail rate exceeded limit '.

When I get this I normally receive an email or see a link in the browser to reset the account but this isn't happening anymore?

Please help.
Thanks, Kevin